moodle.uws.ac.uk

Week 6 Networks
Computing Systems
Lecture 6
Networks
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Week 6 Networks
From Colossus to
World of Warcraft
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Week 6 Networks
• Early computers were standalone
• no communication between different
computers
• Development of terminals allowed multiple
users (possibly in different locations) to use the
same computer at the same time
• Terminal only used for keyboard input and
text output (screen or printer)
• Later generations of terminals added graphics &
local processing (e.g. PLATO IV)
Week 6 Networks
Early Multi-User Computing
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Week 6 Networks
• Computer network developed to link
researchers to the limited number of powerful
research computers in the US
• Not (as often said) to create a network that
would survive a nuclear attack
• Removing the need for separate terminals for
each computer, a single terminal could connect
to any computer on the network
Week 6 Networks
ARPANET
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Circuit vs Packet Switching
Week 6 Networks
• Circuit switching creates persistent link between
endpoints
• Any single node failure breaks the entire link
• Packet Switching breaks message into small
packets, each with information on destination
– Each packet forwarded one hop at a time
– Packets from a single message may take
different routes to destination
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Domain Names
SubSubccSLD ccTLD
Domain Domain
some.user@uws.ac.uk
Week 6 Networks
http://www.uws.ac.uk/
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• Circuit switching creates persistent link between
endpoints
• Any single node failure breaks the entire link
• Packet Switching allows successful
communication even where nodes fail
• Packet Switching breaks message into small
packets, each with information on destination
• Packets from a single message may take
different routes to destination
Week 6 Networks
Circuit vs Packet Switching
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• Data between machines sent in packets
• Large data transfers broken into multiple
packets
• Each packet contains
• IP (Internet Protocol) addresses of destination
& sender
• Packet sequence number
• Data contents
• Cyclic Redundancy Check (CRC) data to verify
correct transmission of packet
Week 6 Networks
Data Packets
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Quick quiz
Week 4 Compilers and
Interpreters
11
• Join the ‘Socrative’ app ‘Room 642124’ and try the quick quiz.
Week 6 Networks
OSI(Open Systems Interconnection model) Seven
Layer Model
• Splits network communications into a set of
different layers:
• Application, Presentation, Session, Transport,
Network, Data Link, Physical
• From user programs (web-browsers and online
games) to the physical links between nodes
• Each uses the layer below, but doesn’t need to
know how the lower level works
• Multi-user game does not need to know how
the machines are actually physically connected,
browser doesn’t know if I’m using WiFi or 3g...
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OSI seven layer Model
Data
•Presentation Layer
•Translates the message into a language that the receiving computer can understand (often ASCII, a way of encoding text as bits). Compresses
& encrypts the data. Adds another header specifying the language as well as the compression and encryption schemes.
Data
•Session Layer
•Opens communications. Sets boundaries for the beginning and end of the message. Establishes whether the message will be sent half
duplex or full duplex. Details are placed into a session header.
Segments
•Transport Layer
•Protects the data being sent. Subdivides the data into segments and creates checksum tests used later to determine whether the data was
scrambled. Makes backup copies of the data. The header identifies each segment's checksum and its position in the message.
Packets
•Network Layer
•Selects a route for the message. Forms segments into packets, counts them, and adds a header containing the sequence of packets and the
address of the receiving computer.
Frames
•Data Link Layer
•Supervises the transmission. Confirms the checksum, and then addresses and duplicates the packets. Keeps a copy of each packet until it
receives confirmation from the next point along the route that the packet has arrived undamaged.
Bits
•Physical Layer
•Encodes the packets into the medium that will carry them-such as an analogue signal, if the message is going across a telephone line-and
sends the packets along that medium.
Week 6 Networks
Data
•Application Layer
•Converts a message's data into bits and attaches a header identifying the sending and receiving computers
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• The first layer, the application layer, is the only
part of the process a user sees, and even then
the user doesn't see most of the work the
application does to prepare a message for
sending over a network.
• The layer converts a message's data into bits and
attaches a header identifying the sending and
receiving computers.
Week 6 Networks
Application Layer
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• The presentation layer translates the message
into a language that the receiving computer can
understand (often ASCII, a way of encoding text
as bits). This layer also compresses and perhaps
encrypts the data. It adds another header
specifying the language as well as the
compression and encryption schemes.
Week 6 Networks
Presentation Layer
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• The session layer opens communications. It sets
boundaries (called brackets) for the beginning
and end of the message and establishes whether
the message will be sent half duplex, with each
computer taking turns sending and receiving, or
full duplex, with both computers sending and
receiving at the same time. The details of these
decisions are placed into a session header.
Week 6 Networks
Session Layer
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• The transport layer protects the data being sent.
It subdivides the data into segments and creates
checksum tests − mathematical sums based on
the contents of data − that can be used later to
determine whether the data was scrambled. It
also makes backup copies of the data. The
transport header identifies each segment's
checksum and its position in the message.
Week 6 Networks
Transport Layer
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• The network layer selects a route for the
message. It forms segments into packets, counts
them, and adds a header containing the
sequence of packets and the address of the
receiving computer.
Week 6 Networks
Network Layer
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• The data-link layer supervises the transmission.
It confirms the checksum, and then addresses
and duplicates the packets. This layer keeps a
copy of each packet until it receives
confirmation from the next point along the
route that the packet has arrived undamaged.
Week 6 Networks
Data-Link Layer
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• The physical layer encodes the packets into the
medium that will carry them-such as an
analogue signal, if the message is going across a
telephone line-and sends the packets along that
medium.
Week 6 Networks
Physical Layer
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• Transmission Control Protocol/Internet Protocol
• TCP is a transport layer protocol
• IP is a network layer protocol
• Each node on the internet has an IP address
www.uws.ac.uk = 146.191.60.100
• Special servers known as Domain Name System
Servers map node names to IP addresses
Week 6 Networks
TCP/IP
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Week 6 Networks
C:\Users\admin>tracert 146.191.60.100
Tracing route to uws.ac.uk [146.191.60.100] over a maximum of 30 hops:
1 67 ms <1 ms 1 ms XXXXXXX-XxX [192.168.2.1]
2 36 ms 40 ms 34 ms xx.xxx.xxx.x
3 24 ms 16 ms 25 ms renf-cam-1b-v125.network.virginmedia.net [80.4.64.245]
4 44 ms 18 ms 44 ms renf-core-1b-ae2-0.network.virginmedia.net [195.182.176.181]
5 40 ms 26 ms 19 ms leed-bb-1b-as3-0.network.virginmedia.net [213.105.175.225]
6 32 ms 36 ms 27 ms leed-bb-1a-ae0-0.network.virginmedia.net [62.253.187.185]
7 27 ms 24 ms 20 ms brnt-bb-1b-as2-0.network.virginmedia.net [213.105.175.26]
8 51 ms 29 ms 28 ms brnt-tmr-1-ae5-0.network.virginmedia.net [213.105.159.50]
9 59 ms 51 ms 56 ms telc-ic-1-as0-0.network.virginmedia.net [62.253.185.74]
10 32 ms 26 ms 18 ms linx-gw2.ja.net [195.66.226.15]
11 69 ms 38 ms 67 ms ae1.lond-sbr3.ja.net [146.97.35.173]
12 33 ms 158 ms 32 ms as0.read-sbr1.ja.net [146.97.33.165]
13 36 ms 29 ms 32 ms so-5-0-0.warr-sbr1.ja.net [146.97.33.90]
14 42 ms 46 ms 46 ms ae14.glas-sbr1.ja.net [146.97.33.122]
15 40 ms 46 ms 41 ms clydenet.glas-sbr1.ja.net [146.97.40.98]
16 46 ms 43 ms 47 ms paisley-ge1-1-glasgowpop-ge8-13-v200.clyde.net.uk [194.81.62.202]
17 32 ms 37 ms 36 ms 146.191.225.1
18 40 ms 67 ms 39 ms 146.191.241.9
19 40 ms 36 ms 32 ms 146.191.241.5
20 34 ms 35 ms 30 ms uws.ac.uk [146.191.60.100]
Trace complete.
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Quick quiz
Week 4 Compilers and
Interpreters
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• Join the ‘Socrative’ app ‘Room 642124’ and try the quick quiz.
• Network of computers in a local area
• E.g. An office, school, university, home...
• Ethernet (IEEE 802.3) is the dominant standard
for local area networks
• A number of machines (computers,
printers, etc) may be connected
in a bus or star network
• Hubs, switches & routers can be
used to connect nodes
Week 6 Networks
Local Area Networks (LAN)
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• Bus
• All nodes connected via
a single cable
• Only one (pair) of
machines can
communicate at a time
• Contention
• Star
• Hub, router or
switch is central
node
• Connects devices
to a single point
Week 6 Networks
LAN Topology
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• Small home & office networks typically connect
to internet via:
• Dial-in: modem uses standard phone line and
exchange. Limited bandwidth.
• DSL (Digital Subscriber Line) requires updated
machines at local exchange to offer higher
speeds over a standard phone line
• Asynchronous DSL: Higher download than
upload bandwidth
• Cable Internet uses cable television
infrastructure to provide internet access
Week 6 Networks
Home and Small Office
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DSL
Cable Internet
• Good availability
• Phone and internet on
one connection
• Transmission rate
affected by distance to
exchange, quality of
phone line, local
exchange equipment
• Big variance in
performance
• Limited availability
• Separate connection
• Cable originally designed
for broadcast – high
use can cause network
congestion
• Typically higher
performance than DSL
Week 6 Networks
DSL vs Cable Internet
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• Wireless networking, IEEE802.11 a/b/g/n
• Commonly built into laptops and now many
home devices (e.g. PS3/4)
• Need nearby access point (e.g. WiFi cable
modem) ~30-50m
• Unencrypted or unprotected WiFi connections
can pose serious security risk
Week 6 Networks
Wireless Internet
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• Wireless transmissions can be ‘heard’ by any
listening device
• Not just the access point
• Unencrypted messages can be easily copied
• WEP: Wired Equivalent Privacy
Deprecated (not recommended for use) – one
encryption key for all packets
• WPA & WPA-2: Wi-Fi Protected Access – uses
a different encryption key for every data packet
Week 6 Networks
Wireless Security
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• Early mobile web-access limited by small displays
and expensive data-transfer rates
• WAP & WML – Wireless Access Protocol and
Wireless Markup Language
• Modern mobile internet on third and fourth
generation networks (3g and 4g) provide higher
bandwidth data rates
• To broadband equivalent rates
• Used in latest generations of Smartphones and for
mobile internet access on laptops
Week 6 Networks
Mobile Internet
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iOS
Android
webOS
Windows BlackBerry
Phone
OS
Company
Apple
Open
Handset
Alliance
(Google)
HP/Palm
Inc
Microsoft
RIM
Samsung
OS Family
Mac OS
X/Unix-like
Linux
Linux
Windows
CE 7
Mobile OS
*nix
ARM
ARM,
MIPS,
Power
Architecture
, x86
ARM
ARM
ARM
ARM
C, C++,
Java
C
Many, .NET
(Silverlight/
XNA)
Java
C++
Open
source +
closed
source
elements
Open
source +
closed
source
modules
CPU
C, C++,
Programmed
Objectivein
C
License
Proprietary
+ open
source
elements
Official
SDK
Mac OS X
platform(s)
Multiplatform
Multiplatform
Bada
Week 6 Networks
Feature
Proprietary Proprietary Proprietary
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Windows
Windows
Windows
• Finding documents or resources on the internet
was not always easy
• Often required detailed knowledge of what
computers were actually available, and where
particular information was likely to be held
• In 1989 Sir Tim Berners-Lee proposed a
hypertext based information sharing system for
the internet
• Hypertext already existed, but not networked
(e.g. Hypercard on Mac)
Week 6 Networks
The World Wide Web
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• Interviewed for The Virtual Revolution, Tim BernersLee explains why he invented the World Wide Web:
• Video Here (Start at 6:05)
• “I invented the web... because it was so frustrating
that it didn’t exist”
• The problem was the lack of a common standard
documentation system that allowed links between
documents – even where the documents lived on
different systems
Week 6 Networks
Tim Berners-Lee: Birth of the
Web
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• 1990: First web-server, http://info.cern.ch
• 1993: Mosaic Browser brought graphical
browsing to Mac & PC computers
• 1995: First edition of Bill Gates’ book “The Road
Ahead” doesn’t mention the web at all
• 1999-2001: DotCom Boom and Bust
• Today: The Ubiquitous Web
Week 6 Networks
A Brief History of the Web
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• HTTP: HyperText Transport Protocol – the
protocol used by browsers and servers to
request and provide pages
• HTML: HyperText Markup Language – mark-up
language used to write web-pages
• URL: Universal Resource Locator – each
document or resource on the internet can have a
unique URL that anyone to access that file
Week 6 Networks
Web Terms
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• The Web brought the internet into daily life and
ordinary homes
• Access to information and services
• Later generations of applications focussed less on
letting users access information, more on letting
users create their own
• Blogs, wikis, social networks, ...
• The read/write web
Week 6 Networks
Web 2.0 and On...
• A good app is IFTTT it links all your social networks together in one
place
• You can create recipes - worthwhile looking at
• https://ifttt.com/wtf
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• One machine on internet acts as a
server
• One or more machines as clients
• Examples:
• Chat or Instant messaging: User runs client
program, which connects to a server run by
AOL/Microsoft, etc
• Some multi-user games require one user to
run as a server (dedicated or as client as well)
• MMO games (World Of Warcraft)
Week 6 Networks
Client/Server
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• Made (in)famous by Napster
& illegal file-sharing
• Many legitimate uses
• Many P2P applications use a subset of peers to
act as directories to allow peers to identify
resources or other peers on the network
• E.g. Skype supernodes
• BitTorrent peers can be both seeders
(providers of files) and trackers (directories)
Week 6 Networks
Peer To Peer
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• Some applications may use maintain
client/server distinction – but use distributed
networks of servers
• E.g. OpenSim Hypergrid
• OpenSim is an open source 3D virtual world
platform: anyone can run a server to create
their own small virtual world
• OpenSim virtual worlds can be linked
• Users can travel between worlds from a single
client
Week 6 Networks
Distributed Server Networks
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Quick quiz
Week 4 Compilers and
Interpreters
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• Join the ‘Socrative’ app ‘Room 642124’ and try the quick quiz.
• Wikipedia:
• History of the Internet, TCP/IP, DSL, History of
the World Wide Web, etc.
• PCH:
• Chapter 14, Computer Communications
Week 6 Networks
Further Reading
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• HCW:
• Chapter 9: p132-143 (Bitmaps & Vector
Images)
• Chapter 19 (Digital Cameras)
• Chapter 20 (Multimedia Sound)
• Chapter 21 (Multimedia Video)
• Chapter 29 (Internet Video/Audio)
Week 6 Networks
Required Reading For Next
Week
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Title Image – CC-BY-SA Daniel Livingstone, 2011
World of Warcraft screenshot – CC-BY Juanpol, http://www.flickr.com/photos/juanpol/8232592/
Mobile phone GPS – CC-BY HawaiianMama
http://commons.wikimedia.org/wiki/File:GPS_on_smartphone_cycling.JPG
Ethernet RJ-45 Connector – CC-BY-SA David Monniaux ,
http://en.wikipedia.org/wiki/File:Ethernet_RJ45_connector_p1160054.jpg
Network Topology: Bus, Public domain, from Wikimedia Commons, Star CC-BY-SA Head,
http://commons.wikimedia.org/wiki/File:Netzwerktopologie_Stern.png
Network Topology/DataFlow CC-BY-SA Kbrose/Cburnett
http://en.wikipedia.org/wiki/File:IP_stack_connections.svg
OSBridge WiFI access point, Public Domain by Rewt, http://en.wikipedia.org/wiki/File:3GN_.jpg
Client/Server network, Gnu LGPL by Maurio Bieg, http://commons.wikimedia.org/wiki/File:Serverbased-network.svg
Peer to Peer Network, Public Domain by Maurio Bieg http://commons.wikimedia.org/wiki/File:P2Pnetwork.svg
Telephone mast, CC-BY-SA Andrew Tatlow,
http://commons.wikimedia.org/wiki/File:Mobile_telephone_mast_-_geograph.org.uk_-_165771.jpg
Smartphone market share, CC-BY-SA Eraserhead1,
http://en.wikipedia.org/wiki/File:Smartphone_share_current.png
Product names, logos, brands, and other trademarks featured or referred to within the these notes are
the property of their respective trademark holders. These notes have been produced without
affiliation, sponsorship or endorsement from the trademark holders.
Week 6 Networks
Image Credits:
This work by Daniel Livingstone at the University of the West of Scotland is licensed
under the Creative Commons Attribution-ShareAlike 3.0 Unported License. To view a
copy of this license, visit http://creativecommons.org/licenses/by-sa/3.0/ or send a
letter to Creative Commons, 171 Second Street, Suite 300, San Francisco, California,
94105, USA.
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